The present invention relates to a user detection circuit for a display apparatus that switches between the normal operation and the power-saving standby operation (hereinafter referred to as standby operation) of the display apparatus by detecting a user in front of the display screen.
Recently, a flat-panel display apparatus such as a liquid crystal display apparatus has been drawing attention as a display apparatus alternative to the CRT. In addition, energy conservation is required of OA equipment in general including this type of display apparatus. Therefore, a flat-panel display apparatus is required to save energy by suspending display and switching itself to standby operation to stand by when display is not necessary.
As a method for automatically switching between normal operation and standby operation in this type of display apparatus, there is a method conceivable in which a user in front of the display screen is detected by a sensor, and the apparatus is switched to standby operation if the user is absent for a certain period of time.
FIG. 7 is a circuit diagram showing a concrete example of such a user detection circuit as a prior art.
A sensor 10 for detecting a user (hereinafter referred to as a user sensor) is a reflection type infrared sensor having a light emitting unit 12 and a light receiving unit 14.
The light emitting unit 12 has a resistance 12A, an infrared diode 12B, and a switch 12C, and emits infrared rays from the infrared diode 12B to the front of a display apparatus.
On the other hand, the light receiving unit 14 has a phototransistor 14A and a resistance 14B. The phototransistor 14A receives the infrared rays which are emitted from the infrared diode 12B of the light emitting unit 12 and reflected from the user, and then a detection signal in response to the quantity of light is outputted.
However, although in such a user sensor 10, the phototransistor 14A has frequency characteristics to detect infrared rays, it is difficult for the phototransistor 14A to distinguish between the light reflected from the user and infrared rays included in the light in the environment that also enter the phototransistor 14A. This may cause erroneous function.
As a measure to deal with this problem, the infrared diode 12B is conventionally made to provide pulsed lighting by turning on and off the switch 12C of the light emitting unit 12, so that a lighting time period and the other time period are each detected by a sample hold circuit consisting of a switch 15 and hold circuits 16 and 18. Then only a reflected component is extracted by subtracting an environmental light component.
Specifically, in FIG. 7, the switch 12C in the light emitting unit 12 and the switch 15 on the side of the light receiving unit 14 open and close with pulse periods in sync with each other. That is, when the infrared diode 12B in the light emitting unit 12 is turned on, the output of the phototransistor 14A in the light receiving unit 14 is inputted to a first hold circuit 16. When the infrared diode 12B in the light emitting unit 12 is turned off, the output of the phototransistor 14A in the light receiving unit 14 is inputted to a second hold circuit 18.
The first hold circuit 16 has a capacitor 16A and a buffer circuit 16B. The capacitor 16A accumulates detection voltage from the phototransistor 14A during a time period when the infrared diode 12B turns on, and then the level of the resulting voltage is outputted via the buffer circuit 16B.
The second hold circuit 18 has a capacitor 18A and a buffer circuit 18B. The capacitor 18A accumulates detection voltage from the phototransistor 14A during a time period when the infrared diode 12B is turned off, and then the level of the resulting voltage is outputted via the buffer circuit 18B.
Therefore, sample hold output from the first hold circuit 16 corresponds to a quantity of infrared rays representing a total of infrared rays reflected from the user and infrared rays from environmental light. Sample hold output from the second hold circuit 18 corresponds to a quantity of infrared rays that does not include infrared rays reflected from the user but includes only infrared rays from environmental light.
Thus, the polarity of the sample hold output from the second hold circuit 18 is reversed by a reversing circuit 20, and then the resulting sample hold output from the second hold circuit 18 is added to the sample hold output from the first hold circuit 16 by an adding circuit 22. A resultant signal is outputted as a sensor output signal via an amplifier 24.
This makes it possible to provide sensor output that corresponds only to a quantity of infrared rays emitted from the light emitting unit 12 and reflected from the user without being affected by increases or decreases in the quantity of environmental light. Then, presence or absence of the user in front of the display apparatus is detected by determining the level of the sensor output using a determination circuit.
In the conventional user detection circuit as described above, however, the operation of the switch 12C in the user sensor 10 needs to be synchronized precisely with the operation of the switch 15 on the side of the light receiving unit 14 by a timing circuit (not shown in the figure) for controlling the switches.
In addition, switching noise should not be mixed into the capacitors 16A and 18B.
Thus, the conventional user detection circuit described above has a complex circuit configuration. Moreover, attention needs to be paid to the arrangement of components and further to the capacities and temperature characteristics of the capacitors 16A and 18B.
Therefore, an object of the present invention is to provide a user detection circuit for a display apparatus that makes it possible to perform highly accurate user detection with a simple configuration without using a highly accurate, complex timing circuit or sample hold circuit.
According to the present invention, in order to achieve the above object, there is provided a user detection circuit that detects absence of the user in front of the display screen and thereby switches the normal operation of the display apparatus to power-saving standby operation for allowing the display apparatus to suspend display and standby. The user detection circuit includes a user sensor for detecting a user in front of the display screen, an illumination sensor for detecting the brightness of the environment of the display screen; a correction means for correcting a detection signal of the user sensor based on a detection signal of the illumination sensor, and a determination means for determining whether the user is present based on the detection signal of the user sensor corrected by the correction means.
In the user detection circuit according to the present invention, the user sensor detects a user in front of the display screen, while the illumination sensor detects the brightness of the environment of the display screen.
The correction means corrects a detection signal of the user sensor based on a detection signal of the illumination sensor. The determination means determines whether the user is present based on the detection signal of the user sensor corrected by the correction means.
Thus, in the user detection circuit according to the present invention, conventional sample hold operation by time division is not required because detection signals are corrected by using a user sensor and an illumination sensor that are independent of each other. Hence a highly accurate, complex timing circuit or sample hold circuit for the above sample hold operation is not required.
It is thus possible to perform highly accurate user detection with a simple configuration, and therefore correctly switch between the normal operation and the standby operation of the display apparatus.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols.